1. Field of the Invention
The present invention relates to the field of vehicle transmissions. It refers to a power-branched transmission, in particular for agricultural vehicles, such as tractors or the like, and to a method for the operation of such a transmission.
2. Description of Related Art
Power-branched transmissions, particularly for use in vehicles employed for agricultural or construction purposes, such as, for example tractors, have been known for a long time. In such power-branched transmissions, the power prevailing at an input shaft or drive shaft and normally output by an internal combustion engine is apportioned to a first mechanical power branch with a fixed ratio and to a second power branch of continuously variable ratio and is subsequently combined again in order to be available at an output shaft or take-off shaft. The second power branch is mostly designed as a hydrostatic branch, in which two hydrostatic axial piston machines (hydrostats) of the oblique axis or swashplate type, which are connected hydraulically to one another, operate selectively as a pump or as a motor. The ratio can in this case be varied by a variation in the pivot angle of the cylinder block or of the swashplate. Apportioning the power to the two power branches and combining the branched powers normally take place by means of a planetary gear. Power-branched transmissions of the type described are disclosed in various embodiments in DE-A1-27 57 300, DE-C2-29 04 572, DE-A1-29 50 619, DE-A1-37 07 382, DE-A1-37 26 080, DE-A1-39 12 369, DE-A1-39 12 386, DE-A1-43 43 401, DE-A1-43 43 402, EP-B1-0 249 001 and EP-A2-1 273 828.
So that a power-branched transmission can be used successfully in practice, it should generally be distinguished by the following properties:                The transmission should have high efficiency over the entire speed range. This should be the case particularly at the high driving speeds which occur in road traction over a lengthy period of time.        The transmission should have a compact construction, in order to allow installation in the most diverse possible vehicles, if possible without structural restrictions.        The transmission should make it possible to transfer high powers.        The transmission should have as simple a construction as possible in order to limit the power losses and increase operating reliability.        The transmission should allow fully comprehensive electronic control in connection with engine management and, even in the event of a failure of specific control elements, make sufficient emergency driving programs available.        
In DE-A1-43 43 402 initially mentioned, a power-branched transmission designated as an SHL transmission (continuously variable hydrostatic power-branched transmission) has already been described, distinguished by two hydraulically coupled identical hydrostats in the oblique-axis type of construction which can be coupled in different ways to a planetary differential gear via pairs of clutches or selective shift elements K1/K2 or K3/K4. The known SHL transmission has been used and tested in town buses under the type designation SHL-Z. The two hydrostats used have a pivoting range of only 0-25°. For forward drive, in this case, there are 3 driving steps or driving ranges: in the first driving range, at the starting point, the hydrostatic fraction of the transferred power is 100% and then approaches zero linearly with the speed. In the second driving range, it goes from zero to a maximum of about 27% and then back to zero again. In the third driving range, it goes from zero to a maximum value of 13% at the highest forward speed.
The known SHL transmission has the disadvantages not only of the subdivision of the forward drive into three driving ranges, which leads to an increased outlay in shift and control terms but, above all, of the hydrostatic power transfer fraction which deviates markedly from zero at maximum speed. This leads, on long-distance trips in which the high speeds are maintained virtually constantly over a lengthy period of time, to unnecessary efficiency losses which have an adverse effect on consumption and on exhaust gas emission.
The object of the invention, therefore, is to provide a continuously variable hydrostatic power-branched transmission which avoids the disadvantages of known transmissions and which is distinguished, in particular, by high and improved efficiency in rapid forward drive, and also to specify a method for the operation of such a transmission.